The present application is related to my co-pending application, entitled "FIRING HEAD FOR A TUBING CONVEYED PERFORATING GUN", Ser. No. 762,171, filed Aug. 5, 1985, and assigned to the assignee of the present invention:
1. Field of the Invention
The present invention relates generally to firing devices of the type used to fire perforating guns used to perforate a cased well bore for the production of well bore fluids and, specifically, to a tubing pressurized firing device for firing a tubing conveyed perforating gun responsive to changes in the fluid pressure within the well bore tubing string.
2. Description of the Prior Art
The integrity of oil well and gas well bores is generally preserved during drilling operations by cementing a casing or liner in place in the bore hole. The casing or liner is a cylindrical conduit which must be punctured or perforated over the desired production interval in order to produce well bore fluids. Various types of perforating guns are known which utilize some form of a fired projectile and an explosive charge to perforate the casing or liner. Prior perforating gun techniques have either utilized tools which were run on a wireline or cable or were tubing conveyed devices which were run on a tubing string to the desired depth in the well bore.
Several problems exist with wireline run perforating gun systems. Wireline methods require a delicate balance between expected formation pressure and the drilling mud or fluid used to provide a balanced hydrostatic head in the well bore. A miscalculation of the expected formation pressure can result in a tangled wireline. If an over balanced pressure condition exists during the perforation step, the well can lose large volumes of fluid into the surrounding formation which can damage the formation. Also, proper pressure differentials are needed to effectively clean the perforations. It is difficult with the wireline system to obtain the pressure differential needed to back surge the perforations and provide a clean well which will produce high yields. It is also difficult to use wireline systems in deviated well bores.
The tubing conveyed perforating gun generally allows immediate safe release of formation pressure at maximum pressure differentials into the tubing string. The back surge which results tends to clean the perforation of mud filtrate, cement, and other perforating debris. In spite of these advantages, prior tubing conveyed perforating guns have lacked the ideal detonating system.
My co-pending application, Ser. No. 762,171 shows a tubing conveyed perforating gun which is actuated by pressuring the tubing interior from the well surface. The perforating gun can be utilized with a hydraulically set packer where the packer is set by pressuring the tubing to a first predetermined level and where the perforating gun is actuated by pressuring the tubing to a second predetermined level. The perforating gun is also provided with a delay mechanism which provides a known delay interval between pressuring the tubing to the second predetermined level and the actual firing of the perforating gun.
In my previous design, pressuring up the tubing string opens an orifice within the tool and drains an oil-filled chamber. This action allows a piston to move upwardly within the chamber and free a spring loaded firing pin. The mechanical action of the spring acting upon the firing pin propels the firing pin in the direction of a percussion detonator to fire the gun.
It is an object of the present invention to provide for hydraulic actuation of the firing pin whereby pressure in the well annulus acts upon the firing pin to propel the firing pin toward the percussion detonator.
Another object of the present invention is to provide a safety mechanism which normally biases the firing pin in a direction away from the detonator so that the gun can only be fired when fluid pressure from the well annulus exceeds a predetermined level.